1. Field of the Invention
The present invention relates generally to a cellular mobile communication system, and in particular, to a method for quickly resuming a downlink service in a hard handover.
2. Description of the Related Art
Typically, a cellular communication system divides its service area into smaller service areas, i.e., cells covered by base stations (BSs). A mobile switching center (MSC) controls these BSs such that mobile stations (MSs) can continue ongoing calls, when moving from one cell to another. The cellular communication system uses FDMA (Frequency Division Multiplexing Access), CDMA (Code Division Multiple Access), etc., to enable a BS to wirelessly communicate with a plurality of MSs.
In a CDMA communication system, radio channels are identified by orthogonal spreading codes in order to share the same frequency and the same time. One of the features of the CDMA communication system is a soft handover for simultaneously connecting an MS in an overlap area between cells to channels of at least two BSs, to guarantee stable communication.
Compared to the CDMA characterized by spreading of data prior to transmission, orthogonal frequency division multiplexing (OFDM), which has recently attracted much interest, inverse-fast-Fourier-transforms (IFFT) data and inserts a guard interval to the IFFT data prior to transmission. As a result, OFDM offers a benefit of transmitting a wideband signal while using relatively simple hardware compared to the CDMA. An OFDM communication system simultaneously delivers modulated symbols at narrowband frequencies called sub-carriers. Because these sub-carriers use very narrow frequency bands, it is considered that they are flat-fading on the whole.
However, the OFDM communication system does not support a soft handover for simultaneously connecting two channels because neighbor cells use different sub-carriers to prevent interference between them. Therefore, an MS is disconnected from a channel of an old cell and connected to a channel of a new cell, although it usually happens too fast for a user to recognize during voice communication. This operation is called a hard handover. Here, the channels use different sub-carriers. The most important thing in performing the hard handover is to prevent a time delay required for the MS to receive a service from the new BS from affecting the service.
In the hard handover, the new BS assigns channels to the MS for transmission/reception of traffic and control information. An ongoing call is interrupted during the channel assignment. To enable both uplink and downlink services for the MS in the hard handover, the MS must access the new BS and get a new local ID, a traffic channel, and a control channel assigned from the new BS. However, a long time delay can be created by the possible collision between the MS and another MS during the access to the BS. During the hard handover, a time delay of tens of milliseconds occurs. In the case of real-time traffic sensitive to delay such as moving pictures, this time delay significantly degrades quality of service.